Jon Bang

Probe calibration for freehand 3-D ultrasound

Ultrasound (US) probe calibration establishes the rigid body transformation between the US image and a tracking device attached to the probe. This is an important requirement for correct 3-D reconstruction of freehand US images and, thus, for accurate surgical navigation based on US. In this study, we evaluated three methods for probe calibration, based on a single-point phantom, a wire-cross phantom requiring 2-D alignment and a wire phantom for freehand scanning. The processing of acquired data is fairly common to these methods and, to a great extent, based on automated procedures. The evaluation is based on quality measures in 2-D and 3-D reconstructed data. With each of the three methods, we calibrated a linear-array probe, a phased-array sector probe and an intraoperative probe. The freehand method performed best, with a 3-D navigation accuracy of 0.6 mm for one of the probes. This indicates that clinical accuracy in the order of 1 mm may be achieved in US-based surgical navigation.

Ability of navigated 3D ultrasound to delineate gliomas and metastases – comparison of image interpretations with histopathology

SummaryBackground. The objective of the study was to test the ability of a 3D ultrasound (US) based intraoperative imaging and navigation system to delineate gliomas and metastases in a clinical setting. The 3D US data is displayed as reformatted 2D image slices. The quality of the displayed 3D data is affected both by the resolution of the acquired data and the reformatting process. In order to investigate whether or not 3D US could be used for reliable guidance in tumour surgery, a study was initiated to compare interpretations of imaged biopsy sites with histopathology. The system also enabled concomitant comparison of navigated preoperative MR with histopathology.Method. Eighty-five biopsies were sampled between 2–7 mm from the tumour border visible in the ultrasound images. Biopsies were collected from 28 operations (7 low-grade astrocytomas, 8 anaplastic astrocytomas, 7 glioblastomas and 6 metastases). Corresponding cross-sections of preoperative MR T1, MR T2 and intraoperative US were concomitantly displayed, steered by the biopsy forceps equipped with a positioning sensor. The surgeons’ interpretation of the images at the electronically indicated biopsy sites were compared with the histopathology of the samples.Findings. The ultrasound findings were in agreement with histopathology in 74% (n = 31) for low-grade astrocytomas, 83% (n = 18) for anaplastic astrocytomas, 77% (n = 26) for glioblastomas and 100% (n = 10) for metastases. Excluding irradiated patients, the results for glioblastomas improved to 80% concurrence (n = 20). As expected tumour cells were found in biopsies outside the US visible tumour border, especially in low-grade gliomas. Navigated 3D US have a significantly better agreement with histopathology than navigated MR T1 for low-grade astrocytomas.Conclusion. Reformatted images from 3D US volumes give a good delineation of metastases and the solid part of gliomas before starting the resection. Navigated 3D US is at least as reliable as navigated 3D MR to delineate gliomas and metastases.

Multimodal Image Fusion in Ultrasound-Based Neuronavigation: Improving Overview and Interpretation by Integrating Preoperative MRI with Intraoperative 3D Ultrasound

Objective: We have investigated alternative ways to integrate intraoperative 3D ultrasound images and preoperative MR images in the same 3D scene for visualizing brain shift and improving overview and interpretation in ultrasound-based neuronavigation. Materials and Methods: A Multi-Modal Volume Visualizer (MMW) was developed that can read data exported from the SonoWand® neuronavigation system and reconstruct the spatial relationship between the volumes available at any given time during an operation, thus enabling the exploration of new ways to fuse pre-and intraoperative data for planning, guidance and therapy control. In addition, the mismatch between MRI volumes registered to the patient and intraoperative ultrasound acquired from the dura was qualified. Results: The results show that image fusion of intraoperative ultrasound images in combination with preoperative MRI will make perception of available information easier by providing updated (real-time) image information and an extended overview of the operating field during surgery. This approach will assess the degree of anatomical changes during surgery and give the surgeon an understanding of how identical structures are imaged using the different imaging modalities. The present study showed that in 50% of the cases there were indications of brain shift even before the surgical procedure had started. Conclusions: We believe that image fusion between intraoperative 3D ultrasound and preoperative MRI might improve the quality of the surgical procedure and hence also improve the patient outcome.

A new method for analysis of motion of carotid plaques from RF ultrasound images

Motion of carotid artery plaques during the cardiac cycle may contribute to plaque disruption and embolism. We have developed a computerized method that objectively analyzes such motion from a sequence of ultrasound (US) radiofrequency (RF) images. A displacement vector map is obtained by 2-D correlation of local areas in consecutive images. From this map, motion dynamics can be quantified and presented as function of time, spatial (image) coordinates or as single numbers. Correct functionality has been verified on laboratory data. Applied to patient data, the method gives temporal results that correlate well with ECG data and the calculated peak systolic velocities of typically 10 mm/s agree well with values reported in the literature. The spatial analysis demonstrates that different plaque regions may exhibit different motion patterns that may cause internal stress, leading to fissures and plaque disruption. Thus, the motion analysis method may provide new and important information about the plaque characteristics and the prospective risk of cerebrovascular events.

Image fusion of ultrasound and MRI as an aid for assessing anatomical shifts and improving overview and interpretation in ultrasound-guided neurosurgery

Abstract We have developed an image fusion module for combined 3D ultrasound and MRI/CT visualization in image-guided neuronavigation. The module demonstrates alternative image visualization possibilities, which might be integrated in future ultrasound-based neuronavigation systems. The image fusion visualization module reads 3D ultrasound, MRI/CT image data sets and patient registration information which are saved during the operation using the SonoWand ultrasound-based neuronavigation system earlier presented by our group [1] . The images are visualized simultaneously in the same scene by overlay, compositing or splitting visualization techniques which makes perception of information easier as compared to visualizing images in different scenes. Quantitation of brain shift between similar structures recognized in both preoperative MRI and intraoperative ultrasound images was done using both a manual and an automatic image processing algorithm. The results show that alternative image visualizations may be useful in order to get immediate feedback regarding brain shift in the early beginning of the operation. The present paper summarizes the experiences gained using this visualization module on patient image data acquired in our clinic during the last year using the SonoWand ultrasound-based neuronavigation system.

A robust and automatic method for evaluating accuracy in 3-D ultrasound-based navigation

We present a robust and automatic method for evaluating the 3-D navigation accuracy in ultrasound (US) based image-guided systems. The method is based on a precisely built and accurately measured phantom with several wire crosses and an automatic 3-D template matching by correlation algorithm. We investigated the accuracy and robustness of the algorithm and also addressed optimization of algorithm parameters. Finally, we applied the method to an extensive data set from an in-house US-based navigation system. To evaluate the algorithm, eight skilled observers identified the same wire crosses manually and the average over all observers constitutes our reference data set. We found no significant differences between the automatic and the manual procedures; the average distance between the point sets for one particular volume (27 point pairs) was 0.27 +/- 0.17 mm. Furthermore, the spread of the automatically determined points compared with the reference set was lower than the spread for any individual operator. This indicates that the automatic algorithm is more accurate than manual determination of the wire-cross locations, in addition to being faster and nonsubjective. In the application example, we used a set of 35 3-D US scans of the phantom under various acquisition configurations. The US frequency was 6.7 MHz and the average target depth was 6 cm. The accuracy, represented by the mean distance between automatically-determined wire-cross locations and physically measured locations, was found to be 1.34 +/- 0.62 mm.

Directional Surveying: Rotating and Sliding Operations Give Different Wellbore Position Accuracy

We present a novel method for analysing the position uncertainty of directional surveys. By this method, the uncertainty can be accurately calculated by conceptually simple analytical formulae. The theory covers any correlation in measurement error and any correlation in toolface, for both stationary and continuous surveys. The results demonstrate the importance of handling the toolface dependence properly. A direct benefit is that too optimistic or overly pessimistic accuracy estimates are avoided. Furthermore, the difference between rotating and sliding operation is significant for many error terms. For example, a misalignment error typically causes 10 times higher position uncertainty in sliding mode than in rotating mode, for a 3000 m long survey. These findings should have implications for the future specification of survey instrument operation procedures, and thus the design of directional survey programs.

Computer simulations of bistable dynamics of a two-mode laser with symmetric or asymmetric gain conditions

The mode hopping dynamics of a two-mode laser are investigated through numerical simulations. The authors derive an algorithm, based on autocorrelation techniques, by which the average mode hopping rate can be estimated from time sequences of the numbers of photons of the two modes. The method works both for symmetric and asymmetric gain conditions. The results show that the average mode hopping rate decreases with increasing asymmetry in the gain coefficients of the two modes. These results are in agreement with what one would expect from physical considerations. In the case of symmetric gain conditions, the results are compared to theoretically derived formulas, showing reasonable agreement in current dependence and in the dependence on the gain saturation coefficients. >

Neurosurgical biopsies guided by 3D ultrasound - comparison of image evaluations and histopathological results

The imaging of gliomas and metastases by 3D ultrasound has been evaluated by comparing image findings and histopathological results. Biopsies have been taken prior to resection and close to the tumour border as shown in the ultrasound images. The tumour border shown in the ultrasound images was verified by an edge detection algorithm using the Sobel method. Ultrasound images, with the biopsy position indicated, have been evaluated and compared to the histhopathology of the corresponding biopsy. The degree of match between image evaluations and histopathology should reflect the ability of 3D ultrasound to image size and location of tumours correctly. The image slices of intraoperative 3D ultrasound are also compared to corresponding image slices of preoperative MR data (Tl and T2). The preliminary results show that intraoperative 3D ultrasound provides accurate information about tumour border and location and may accordingly be used for guiding surgical procedures. For gliomas the histopathologic match for the intraoperative 3D ultrasound is better than the corresponding match for the preoperative MR data.